Projection-type image display apparatuses having a screen and a projection apparatus which projects light beams of an image onto the screen have been widely used. In a typical projection-type image display apparatus, a two-dimensional image is created as a basic image using a spatial light modulator such as a liquid crystal microdisplay or a DMD (Digital Micromirror Device), and then the two-dimensional image is magnified and projected onto a screen using a projection optical system, so that an image is displayed on the screen.
As projection apparatuses, various types including a commercialized product called an “optical projector” have been proposed. In general optical projectors, a spatial light modulator such as a liquid crystal display is illuminated using an illumination device having a white light source such as a high pressure mercury lamp to obtain a modulated image, and the obtained modulated image is magnified and projected onto a screen through lenses. For example, Japanese Patent Laid-Open No. 2004-264512 discloses a technique to divide white light generated by a supper-high pressure mercury lamp into three primary color components R, G, and B with a dichroic mirror, guide these light beams to spatial light modulators corresponding to the respective primary colors to create modulated images, and combine the created modulated images corresponding to the respective primary colors by a cross dichroic prism, to project the images onto a screen.
However, high intensity discharge lamps such as high pressure mercury lamps have a short lifecycle and when they are used for optical projectors or the like, they need to be frequently replaced. In addition, since a relatively large optical system such as a dichroic mirror is needed to extract light beams of the respective primary color components, there is a problem in that the entire system becomes larger.
In order to cope with such problems, a system using a coherent light source such as a laser has also been proposed. For example, semiconductor lasers which have been widely used in industries have a very long lifecycle in comparison with high intensity discharge lamps such as high pressure mercury lamps. In addition, since semiconductor lasers are capable of generating light of a single wavelength, a spectroscopic apparatus such as a dichroic mirror is unnecessary, so that there is an advantage in that the entire system becomes smaller.
On the other hand, in the system using a coherent light source of laser beams or the like, there is another problem in that speckles are generated. Speckles are a spotted pattern which is formed when a coherent light beam such as a laser beam is emitted to a scattering plane. If speckles are generated on a screen, they are observed as spotted luminance unevenness, i.e. brightness unevenness, thus becoming a factor of having physiologically adverse affect on an observer. It is considered that the reason why speckles are generated in the case of using coherent light beams is that coherent light beams reflected from respective portions of a scattering and reflecting plane such as a screen have very high coherency so that coherent light beams interfere with one another to generate speckles. For example, a theoretical review of the generation of speckles is made in detail in Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006.
As discussed above, in the system using a coherent light source, since there is a problem of generation of speckles unique to the coherent light source, techniques for suppressing the generation of speckles have been proposed. For example, Japanese Patent Laid-Open No. 6-208089 discloses a technique in which a laser beam is emitted to a scattering plate, scattered light beams obtained therefrom are guided to a spatial light modulator, and the scattering plate is driven to rotate by a motor, thus reducing speckles.